Nucleophilicity

Nucleophilicity is the kinetic tendency of a species to donate a pair of electrons to an electron-poor center, enabling bond formation. It reflects how readily a molecule or ion acts as a nucleophile in reactions such as substitutions and additions. Nucleophilicity is a kinetic concept and is not identical to basicity, which describes proton affinity; a species can be a strong base but, in some solvents, not a strong nucleophile, and vice versa.

Factors that influence nucleophilicity include charge, atom holding the lone pair, solvent, polarizability, and steric hindrance. Negatively charged species are generally more nucleophilic than their neutral counterparts. The atom bearing the lone pair matters: nucleophilicity typically decreases across a period as electronegativity increases and lone-pair availability changes; down a group, trends depend on the solvent. In protic solvents, smaller anions are heavily solvated and thus less nucleophilic, so nucleophilicity often increases down the group (I− > Br− > Cl− > F−). In polar aprotic solvents, solvation is weaker and nucleophilicity tracks basicity more closely, often favoring smaller, more basic ions (F− > Cl− > Br− > I−). Polarizability also matters: more polarizable (soft) nucleophiles can be more reactive toward soft electrophiles. Steric hindrance reduces nucleophilicity by obstructing approach to the electrophilic center.

Nucleophilicity is a governing factor in reaction rates, especially for SN2 processes, where rate is proportional to the nucleophile concentration and its reactivity toward the electrophile. In SN1 reactions, nucleophilicity plays a smaller role, as the rate-determining step forms a carbocation, with solvent effects and carbocation stability dominating.

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